1. Field of the Invention
This invention relates to a thermoforming method and apparatus which enables the continuous feeding and thermoforming of a web of thermoplastic material into parts produced intermittently by the thermoforming apparatus.
2. Description of Related Art
In our Australian patent application number 13236/88 we disclosed a method and apparatus whereby a heated web of thermoplastic material can be fed continuously into a intermittently operating thermoforming machine. In accordance with this disclosure, a partly cooled web is allowed to remain on a conveyor continuously feeding the web into the entry of a thermoformer, until the cooled surface layer of the web in contact with the conveyor has re-heated to a thermoformable temperature below that at which the web will stick to the conveyor. The disclosure of specification 13236/88 is hereby incorporated by reference into this specification.
Specification 13236/88 suggests various methods and combinations of apparatus by which the continuous feeding of the web can be reconciled with the intermittent operation of the thermoformer. In one such disclosure the conveyor feeds the web into the entry of a thermoformer having vertically opposed molding tools. The length of conveyor carrying the web is extendable and the conveyor exit support roller from which the web leaves the conveyor is extendable between the opposed faces of the molding tools. When the conveyor carrying the web has been fully extended between the opposed tool surfaces, the tool surfaces and the conveyor are moved forwardly in the material flow direction at the same speed while the upper mold tool is lowered onto the hot web and vacuum is applied to engage the web and remove it from the conveyor. After removal of the web from the conveyor surface, the conveyor is retracted and the mold tools are closed to form the required parts from the web. However, forward movement of the web and the mold tools continues without interruption and the exit support roller of the conveyor follows the closed mold tools until forming and cooling has taken place. The mold tools are then separated and the web with the formed parts is ejected from the upper mold tool. The lower mold tool is moved to a position below the conveyor during ejection of the formed web and the upper and lower mold tools then return for the cycle of operations to recommence. The tools may move together or sequentially.
The removal of the web from the conveyor to the upper mold tool by applied vacuum is difficult to execute and may restrict the overall utility of the apparatus because of problems associated with suction plate design and vacuum control required to cope with a melt phase thermoplastic web. It is therefore an object of the present invention to provide an improved thermoforming method and apparatus enabling continuous feeding of a thermoformable web to a thermoformer in which the molding operations occur intermittently.